The present invention relates to a solid stick external analgesic composition. More particularly, this invention relates to physically and chemically stable solid stick compositions for the topical administration of salicylate esters.
Externally applied analgesic compositions are useful in providing relief from minor pain and discomfort in muscles and joints. Such external analgesics may exhibit topical analgesic, topical anesthetic, topical antipruritic, or counterirritant effects. See P. Skierkowski and N. C. Lublanezki, "External Analgesic Products," in Handbook of Nonprescription Drugs, 7th ed.; American Pharmaceutical Association and the National Professional Society of Pharmacists: Washington, D.C., 1982; pp. 513-523. Because counterirritants abate the perception of pain and induce a mild inflammatory reaction, thereby producing a feeling of warmth, counterirritant drugs are commonly used external analgesics. Since methyl salicylate, also known as oil of wintergreen, is considered safe and effective and has a pleasant aroma, it is the most widely used topical counterirritant. External analgesic products are commercially available in various fluid forms, such as lotions, ointments, creams, gels, sprays, and the like, and are typically formulated with other active components, such as menthol, methyl nicotinate, camphor, and the like. See, e.g., Skierkowski and Lublanezki, pp. 521-523. Since these fluid forms are typically applied with an applicator or with the fingers, a chemically and physically stable solid stick form would be neater and more convenient.
Solid sticks are well-established delivery forms for topically applied drugs and cosmetics, being particularly useful for lipsticks, deodorants, and antiperspirants. In general, a solid stick consists of an essentially solid matrix that serves as the base for some active ingredient or cosmetic substance. Typical deodorant sticks, which contain ingredients to mask odors or eliminate their cause but which do not contain an antiperspirant agent, are in effect ethanol gelled With sodium stearate. Since antiperspirant sticks incorporate an astringent, generally aluminum chlorohydrate, that tends to be inactivated under alkaline conditions, the ethanol-stearate formulations used in deodorant sticks are generally unsatisfactory for antiperspirant sticks. Using other lower alcohols or other metal stearate salts will not significantly reduce the problem. Modern antiperspirant stick formulations avoid the alkalinity that inactivates the astringent by employing a hydrophobic waxy matrix. To improve sensory perception and facilitate smooth application, various additives, particularly volatile silicones such as cyclomethicone (silicone VS-7158), are also included in the formulations. See (1) H. E. Jass, "The History of Antiperspirant Product Development," Cosmetics and Toiletries, 95, 25-31 (1980); and (2) G. S. Kass, "Deodorant and Antiperspirant Formulary," Cosmetics and Toiletries, 95, 57-72 (1980). See also U.S. Pat. Nos. 4,425,328, 4,280,994, and 4,229,432 for examples of antiperspirant stick compositions.
External analgesics, including methyl salicylate, have been formulated in solid stick form using a stearate-alcohol-water matrix. See, e.g., Japan Kokai Patent No. 56-53611. To eliminate possible alkaline degradation of methyl salicylate and concomitant release of methanol by transesterification (due to ethanol) or by hydrolysis (due to moisture), it would be more desirable to formulate the stick using a less reactive matrix. Therefore to avoid such alkaline degradation of salicylate esters it is an object of the present invention to provide an external analgesic solid stick composition containing salicylate esters incorporated into a non-reactive hydrophobic waxy matrix.